How do distributed databases provide fault tolerance during network failures?

Distributed databases provide fault tolerance during network failures through three primary mechanisms: data replication, consensus protocols, and automatic recovery processes. These systems ensure that even if parts of the network become unavailable, the database remains operational and data remains consistent and accessible. By distributing data across multiple nodes or regions, these databases minimize single points of failure and enable continued operation during disruptions.

First, replication is a core technique. Data is copied across multiple nodes, so if a network partition isolates some nodes, others can still serve requests. For example, in Apache Cassandra, data is replicated across a configurable number of nodes (called a replication factor). If a network failure disconnects one node, the database can route queries to replicas in available nodes. To maintain consistency during writes, systems often use quorum-based consistency levels. A write operation might require acknowledgment from a majority of replicas (e.g., 3 out of 5 nodes) before it’s confirmed. This ensures that even if some nodes are unreachable, the write is durable on enough replicas to survive the outage. Similarly, read requests can fetch data from the nearest available replica, reducing latency and bypassing disconnected nodes.

Second, consensus protocols like Raft or Paxos help maintain consistency during network partitions. These protocols ensure that all nodes agree on the state of the database even when communication is interrupted. For instance, MongoDB’s replica sets use a Raft-like protocol to elect a primary node. If the primary becomes unreachable due to a network split, the remaining nodes vote to elect a new primary from the available replicas. This allows writes to continue on the new primary while the old primary is isolated. Systems designed for partition tolerance (as per the CAP theorem) prioritize consistency or availability based on their configuration. For example, CockroachDB uses the Raft protocol to ensure strong consistency across regions, while Apache Cassandra allows temporary inconsistencies during partitions but resolves them later using mechanisms like hinted handoffs.

Finally, automatic failure detection and recovery minimize downtime. Distributed databases continuously monitor node health via heartbeats or timeouts. If a node stops responding (e.g., due to network failure), the system marks it as offline and reroutes traffic. Tools like Kubernetes or cloud-native load balancers often integrate with these databases to redirect client requests to healthy nodes. For example, Amazon DynamoDB uses automated multi-region replication: if a region loses connectivity, traffic shifts to another region with up-to-date replicas. Additionally, some systems retry failed operations once connectivity is restored or merge conflicting updates using conflict-free replicated data types (CRDTs). These processes ensure minimal manual intervention and seamless recovery after network issues resolve.